Tires have previously been proposed which have treads of a rubber composition which contains hydrophilic silica or which contains hydrophilic silica which has been hydrophobated.
A significant aspect of this invention is to provide a tire tread of a rubber composition which contains a combination of hydrophilic precipitated silica and hydrophilic precipitated silica which has been hydrophobated in situ within the elastomer host. In other words, such tire tread rubber composition contains dual silica moieties in a sense of containing both hydrophilic precipitated silica and in situ hydrophobated precipitated silica.
In one embodiment, the hydrophobated precipitated silica is a hydrophilic precipitated silica which has been hydrophobated by treatment with, for example, at least one of an alkoxysilane, organoalkoxysilyl polysulfide (silica coupling agent) and organomercaptoalkoxysilane (silica coupling agent), preferably with an organoalkoxysilyl polysulfide or an organomercaptoalkoxysilane coupling agent, optionally including an alkoxysilane, in situ within the rubber composition.
The hydrophilic precipitated silica is a precipitated silica which has not been hydrophobated with a hydrophobtaining agent, particularly not hydrophobated with a silane based hydrophobtaining agent, particularly a siloxane-containing hydrophobtaining agent for the hydrophilic precipitated silica.
In practice, it is considered herein that the rubber reinforcement contribution of such in situ hydrophobated precipitated silica is different than a rubber reinforcement contribution of the hydrophilic precipitated silica, particularly when the precipitated silica has been hydrophobated with said organoalkoxysilyl polysulfide or organomercaptoalkoxysilane coupling agents.
Historically, synthetic amorphous precipitated silica is typically hydrophilic (water loving) in nature and therefore not readily compatible with diene-based elastomers in rubber compositions in general. For this reason, for rubber compositions which contain a significant precipitated silica content, it is often desirable to hydrophobate the precipitated silica to make it more compatible with diene-based elastomers in a rubber composition for a tire tread.
For this invention, while the mechanism is not fully understood, it has been observed that use of a combination of dual precipitated silica moieties, namely a combination of in situ hydrophobated precipitated silica (particularly when hydrophobated in situ with an organoalkoxysilyl polysulfide or organomercaptoalkoxysilane coupling agent) and hydrophilic precipitated silica, in a diene-based elastomer tire tread composition can beneficially provide a tire tread with one or more physical properties which are different than when using only such hydrophobated precipitated silica or when only using a hydrophilic precipitated silica. For example, it has been observed that a tire tread running surface having a relatively high wet coefficient of friction can be obtained by using such combination of dual silica moieties as compared to a tire tread containing a significant in situ hydrophobated precipitated silica content (particularly when hydrophobated in situ with such silica coupling agent) without the presence of a hydrophilic precipitated silica. Apparently the silica coupling agent in situ treated precipitated silica acts to enhance desirable physical properties of the tire tread rubber composition, whereas the hydrophilic precipitated silica does little to enhance the tread rubber physical properties in the manner of the hydrophobic precipitated silica but, instead, particularly enhances the wet coefficient of friction of the tread rubber running surface in a manner which is significantly better than obtained with the hydrophobated silica.
Accordingly, while the mechanism may not be completely understood, it is envisioned that the hydrophobated precipitated silica enhances desirable physical properties and that the hydrophilic (“water loving”) precipitated silica can beneficially enhance an increase in wet traction of the tread rubber surface and therefore is beneficial for a tire tread in a sense of traction performance for wet driving conditions.
It is considered herein that use of such dual silica moieties in a tire tread is novel and a departure from past practice.
A challenge is therefore presented as to how to suitably obtain a combination of such dual silica moieties in a tire tread rubber composition.
In the description of this invention, the term “phr” relates to parts by weight for a material or ingredient per 100 parts by weight elastomer(s)”. The terms “rubber” and “elastomer” may be used interchangeably unless otherwise indicated. The terms “cure” and “vulcanize” may be used interchangeably unless otherwise indicated.